The demetallization of metallized films is a known process. It is a technique used in the packaging industry to create patterns of metallized areas on various packaging substrates. It is also used in the holographic security industry as a technique to enhance the security-providing attributes of holographic films and laminates. Other applications include flexible circuit board design, flexible antennas for RFID tags and similar patch style antennas, wavelength-specific electromagnetic shielding, and others where demetallization serves as a method to achieve desired functionality or graphic qualities. The technology described herein relates to the demetallization of holographic webs, but the disclosed technology is in no means limited to holographic webs only. Non-holographic metallized webs can be effectively processed using the disclosed technology, as well as substrates that are processed in sheet or other non-continuous forms. For simplification, the description is limited to metallized holographic webs. It should be understood that the security and other features created by the application of the technology to holographic webs can also, in part, be realized with the application of the technology to non-holographic materials as well.
The demetallization of metallized holographic films is used in applications where it is imperative that the holographic material be nearly impossible to duplicate by unauthorized persons. Such holographic films and products are used in currencies, security identification cards, passports, tamper indicating labels, various documents of value, and similar products. Importantly, demetallization of holographic overlaminates and other products creates a degree of transparency in the holographic substrate itself as a direct function of the amount of metal removed, further modified by the “negative” shape or pattern created by the removal of the opaque, reflective metal layer. This technique effectively counteracts physical contact copying of holograms due to the localized differences in diffraction efficiency and also provides see-through and other capabilities as is described in U.S. Pat. Nos. 5,044,707, 5,128,779, 5,142,383, 5,145,212 and 5,411,296, each of which is incorporated herein by reference in its entirety.
For example, if on a metallized holographic substrate the metal coating were to be removed in a fine checkerboard or other appropriate pattern, 50% of the substrate would remain reflective and 50% of the substrate would become effectively transmittive, on average. Although the holographic microstructure itself has not been removed in these transmittive areas, the impinging light is diffracted with such lower efficiency than it is in those areas that retain the reflective metal backing that there is no discernible trace of the holographic image. Under close examination it is seen that the holographic surface is still intact but since the entire holographic surface is typically brought into contact with an adhesive layer which adheres the hologram to the underlying document, the similarity of the diffractive indices of the non-metallized holographic surface areas and the adhesive layer serve to optically eliminate the diffractive surface. This is often referred to as “indexing out” a surface. Therefore, due to the elimination of the reflective and diffractive means in 50% of the active area and the preservation of the reflective and diffractive means in the remaining 50%, when such a demetallized film is overlaid on a document the holographic information and the document surface information are both simultaneously visible. In such an application, it is desirable to have at least some areas with a sufficiently fine checkerboard pattern so as to be below or near the resolution threshold of the human eye, or at least of a mean constituent size no greater than the smallest discrete object of information on the document. This prevents unwanted obscuring of document data with relatively large areas of opacity on the holographic overlay. With demetallized holographic overlay devices, still higher levels of document security can be obtained by creating very fine, anisotropic designs such as guilloche, text, company logos, or any other pattern that would be difficult to imitate due to its very high resolution and spatially localized diffractive effect. Another level of security can be achieved with the inclusion of demetallized watermark patterns. The widely accepted technique of holographic film demetallization consists of the following:
First, a holographic image is imparted onto or into a substrate using known means. This would, for example, consist of coating a web of polyester with an ultraviolet radiation-curable varnish and bringing a nickel holographic shim into contact with the varnish while a UV light cures, or hardens the varnish. The nickel holographic shim, being a transmission hologram created originally in photoresist, has on its surface a holographic microstructure that is cast into the varnish as the varnish is cured while in contact with the shim. This process is done repeatedly and/or continually on the web of polyester.
Continuing with the example, the roll of varnish-coated polyester is then coated with a thin layer of aluminum in a vacuum chamber to an optical density of approximately 1.8. The aluminum is applied directly to the holographic side of the web and conforms exactly to the microstructure, creating a reflecting transmission hologram that has very high diffraction efficiency.
This roll of metallized, holographic polyester is then printed with a patterned coating resist, where “resist” can be any form of a substance that is unreactive with corrosive (caustic or acidic) solutions or environments. In the case of an aluminum metallization, the resist must be especially non-reactive to caustics, such as NaOH. The printed design is typically applied to the metallized web with a flexographic, gravure, or lithographic printing technique. The design, then, like printed patterns on other materials, repeats with a certain frequency. The print cylinder circumference or more generally, the length of the print plate in the web direction typically dictates this frequency.
The printed web is then brought through a corrosive bath, such as a diluted NaOH bath at an elevated temperature, where the aluminum is etched from the surface only where there is no local protective overlaying of resist. The web is then brought through at least one rinse bath where the excess caustic is removed and neutralized (in the case of a NaOH bath, an HCl solution will produce salt water as an end agent when mixed proportionately with the NaOH).
Once the caustic has been neutralized and removed from the web, there is left behind a pattern of aluminum that exactly represents the footprint of the printed resist pattern. The resist pattern typically remains on the web. The demetallized, holographic web is then further processed as necessary.
The above technologies and similar variations are known and used in the security holographic industry to create difficult-to-counterfeit documents. Described hereinbelow is an inventive improvement upon the aforementioned and similar technologies. The improved technology is not limited to a pattern of demetallization that must continually repeat as a function of the printing plate or device. The improved technology is, in fact, continually variable.